This paper proposes the design and implementation of Sip Agent, which meet IMS(IP Multimedia Subsystem) specifications based on OSGi(Open Services Gateway Initiative). It uses the Equinox, which implements OSGi specification as the core framework, combined with JAIN-SIP, as the underlying protocol stack, and can meet most of the IMS application. This paper includes the explanation of the logical structure, the main module's structure, the introduction of key technology, and the development of a call procedure using Sip Agent, to prove its availability. The applications, which is built on Sip Agent presented in this paper, has low coupling, high cohesion, easily extensible, high-performance features, and it can build SIP-related application quickly and easily.